The present invention relates to an electromagnetic pilot type check valve, and more particularly to an electromagnetic pilot type check valve of large diameter.
In a conventional electromagnetic valve of large diameter such as shown in Japanese Utility Model Application Publication No. 47-30980 dated Sept. 18, 1972, a pilot operation system is employed. In a check valve mounted into a supply pipe, in order to open a piston type main valve 1 as shown in FIG. 1, a plunger 2 is attracted by an electromagnetic force to open a pilot port 3 provided in main valve 1 so that a fluid existing above (with respect to the drawing) or in the piston cup 1a of the main valve 1 is discharged from the pilot port 3 into a discharge port 35 provided in the fluid pipe. The fluid in an intake port 34 is introduced to the piston cup 1a of the main valve 1 through a pressure balancing port E provided in the main valve 1 and a gap C between the valve body lands of the main valve 1. Since the cross sectional area of the pilot port 3 is greater than the sum of the pressure balancing port E and gap C, the discharge amount of fluid is greater than intake. Therefore, the pressure in the piston cup 1a of the main valve 1 is lower than that at the intake port 34 and the main piston valve 1 is moved upwardly due to the difference between these pressures so as to open the main valve port 36.
When an electromagnetic coil is turned off, the pilot port 3 is closed by the plunger 2 to allow the fluid to be admitted to the piston cup of the main valve 1 through the pressure balancing port E and gap C. At this time, the pressure in the piston cup 1a of the main valve 1 balances that at the front surface thereof, and a valve port 36 is closed by the weight of the main valve 1 itself.
When the diameter of the main valve port 36 is increased, the diameter of valve seat or main valve 1 is necessarily increased, and the area of the inner surfaces of the valve body lands and main piston 1 which define the gap C is also increased even when the distance between the inner surfaces of the valve body lands and main piston 1 is not changed. When the area of the inner surfaces of the valve body lands and main piston 1 which define the gap C is increased, it is necessary that the cross-sectional area a of the pilot port 3 which serves to cause difference in pressure between the rear and front surfaces of the main piston 1 be greater than the area of the pressure balancing port E and gap C.
The plunger 2 is moved when the electromagnetic force F is greater than P.sub.1 .times.a+weight of plunger, wherein P.sub.1 represents the difference in pressure between the inlet and outlet for the fluid, so as to open the pilot port 3. As may be clear from the above, the attraction force for opening the pilot port 3 is increased when the cross-sectional area a of the pilot port 3 is increased. In such a case, a coil having a large electromagnetic force is required. Namely, when a main valve having an increased diameter is employed in a conventional electromagnetic pilot valve, a coil having a large electromagnetic force has to be used. Using such a coil is contradictory to the improvement of an electromagnetic pilot valve in which a main valve of a large diameter has to be opened and closed with a coil having a small magnetic force.